Conventional multilayer materials for use in, for example, substrates for circuit boards, have traditionally consisted of alumina substrates having tungsten and molybdenum conductors, and fired at approximately 1600.degree. C. in a hydrogen atmosphere. Indeed, it is commonplace for such substrates to be fabricated from ceramics such as alumina, forsterite, steatite, cordierite, mullite, and the like. However, due to the very high temperatures necessary to sinter such substrates, it is often necessary to use high melting temperature metals, such as molybdenum and tungsten. Unfortunately, both molybdenum and tungsten have poor electrical conductivity properties which make them less satisfactory for high speed, complex radio frequency ("RF") circuitry. Moreover, in order to achieve adequate densification of materials such as alumina, sintering times of 48 hours or more at high temperature are required.
The current trend in ceramic materials is toward lower cost material processing, a choice of dielectric constants in those materials, low firing temperatures, adjustable/low coefficient of resonant frequency, and low loss. There are several materials properties of interest for use in multilayer RF devices. Low loss, i.e., a high Q material is a property which helps the overall performance of the device. An electrical Q value greater than 500 is desirable. A coefficient of resonant frequency ("T") at or near 0 or alternatively an adjustable T.sub.f is a characteristic which is an important feature for RF filter design and can determine how much the resonant frequency of a filter made using said material shifts with a change in temperature. Accordingly, a T.sub.f goal of less than .+-.10 parts per million per .degree. C. is required for many filter applications. A low firing temperature is also important, as it allows for cofiring silver metallization and hence provides a higher metal Q and a less expensive final device with enhanced RF properties. The typical cofiring temperature for silver metallization is less than 950.degree. C. These requirements when combined with a dielectric constant between 6 and 12, yield a material which is ideal for many RF device applications.
A further limitation of the prior art is that such materials cannot be made using a self limiting chemical reaction to produce a high Q ceramic. The combinations of features described above have heretofore been unavailable in any multilayer materials system. This is due to the fact that the use of a self limiting chemical reaction has not been used to make multilayer ceramic substrates. In contrast, prior art has used essentially non-interacting materials to achieve ceramic materials used in multilayer applications. Examples of such materials can be found in, for example, U.S. Pat. No. 4,939,106 to Miekosa, et al, U.S. Pat. No. 4,654,095 to Steinberg, and U.S. Pat. No. 4,752,531 also to Steinberg.
Accordingly, there exists a need to provide a multilayer ceramic material and methods of making those materials and/or multilayer substrates that demonstrates the characteristics necessary for economical use in RF device applications while avoiding the limitations inherent in the prior art. Moreover, the ceramic material must be easily and repeatably fabricated, and amenable to manufacturing methods that produce materials with consistent electrical and mechanical properties.